Friday, October 26, 2012

Fat knees and firm bottoms

I am always surprised when I discover some one has read my blog, but last weeks reader left a comment which made me realise how little I think about an audience.  The question was "why is a stiffer boat better" and "some flex would be a good thing".  That's correct, some flex is a good thing.  A flexible structure will absorb impact loading softening a blow.  My concern, however, is that localised impact loading from the trailer could be delaminating my bottom planks.  A stiffer bottom and more rigid hull will help distribute the impact load across a broader area instead of the immediate impact zone.

Rubbing strips added to plank edges

It kind of makes sense to me and I think it will help my boat deal with rough conditions.  I hope it makes sense to my reader. I don't have an animation prepared, but this is a real simulation from the "MechanicalLaboratory" you tube channel explains torsional stresses better than I can.

Friday, October 19, 2012

Double the Swiss cheese please

After giving the hull a good going over looking for splits and cracks I pulled off the delaminated sections and sanded back to clean wood.  The majority of the time since then I have been wondering what to do.  Having gone this far my quick clean up in time for my traditional November sail has turned to the major refit I have pondering for the last five years.

To fix the delamination I have decided to do away with the split floor bearer and laminate 4mm ply backing planks giving me a 10mm bottom.  I will also add rubbing strips to the plank edges on the outside of the hull extending across the internal floor bearers.  I'm hoping this will help spread the load from any localised forces on the trailer. I will also replace the old 25mm knees with thicker 40mm knees and instead of screwing the pieces together I will try using stainless bolts and spring washers.  I'm hoping this will help me make the boat stiffer and reduce the tendency for the ply to split.

While all this has been going on I have had an extra reef point added to my mainsail and I am thinking of adding a small foredeck just like this one on vivierboats site.  This should also help stiffen the hull as well as keep some of the spray out.

Friday, October 5, 2012

Swiss cheese

So far the boat has been disassembled and the spars cleaned and oiled without problems.  Damage to the centre board has been repaired without major drama and the trailer is in good order.  When I started looking at what I thought were paint cracks in the hull things went pair shaped.  In a classic example of the swiss cheese model a whole range of factors seem to have conspired to delaminate a patch of my 6mm ply hull.

Most of the floor bearers span the width of the bottom except, for obvious reasons, the bearers either side of the centre board trunk.  Floor bearers, epoxy and ply all flex with the load but the weak point is the bond between layers of ply.  What started as a paint crack developed into delamination and accelerated with the ingress of water and sand.

So out come the offending floor bearers.
What was that I said? "I don't want to spend my summer refitting my boat".